Extended surfactants have shown great potential in enhanced oil recovery (EOR). In this paper, the effects of extended surfactant concentration, polypropylene oxide (PO) number, and various counter ions (Na⁺, Ca²⁺, and Mg²⁺) on the oil-water interfacial tension (IFT) of extended surfactant solutions were investigated in detail. The selected extended surfactants were sodium dodecyl polyoxypropylene sulfate (L-C₁₂PO_mS, m=5, 10, 15). Based on our experimental results, under NaCl conditions, with an increase in the PO number, the space occupied by the surfactant molecules on the side of the oil phase increases, which matches the size of the anionic head, and the surfactant film becomes tight. An ultralow IFT value appears for L-C₁₂PO₁₅S at 0.5% NaCl against decane. The addition of bivalent cations can significantly reduce the IFT. As the Ca²⁺ concentration increases, IFTs of APS solutions against alkanes can achieve ultralow values at lower PO numbers and against alkanes with a wide ACN range. For the MgCl₂ system, the smaller hydration radius of Mg²⁺ improves the tightness of the anionic head, resulting in a crowded arrangement of the PO chains on the oil phase side. Therefore, the ability of Mg²⁺ to reduce the IFT against alkanes is weaker than that of Ca²⁺. Moreover, the surface-active substances in the crude oil adsorb competitively on the interface, resulting in an increase in IFT. By adding Ca²⁺or Mg²⁺, a tight adsorption film of surfactant molecules will be formed, and the IFT value reaches ultralow against crude oil. The studies in this paper have important implications for the application of extended surfactant solutions to enhance oil recovery under high salt reservoir conditions.

延长的表面活性剂在提高石油采收率 (EOR) 方面显示出巨大的潜力。在本文中，扩展的表面活性剂浓度、聚环氧丙烷 (PO) 数和各种反离子（Na ⁺、Ca ²⁺和 Mg ²⁺）对扩展表面活性剂溶液的油水界面张力（IFT）的影响是进行了详细调查。所选的扩展表面活性剂是十二烷基聚氧丙烯硫酸钠 (LC ₁₂ PO _mS，m=5、10、15）。根据我们的实验结果，在NaCl条件下，随着PO数的增加，油相侧表面活性剂分子所占的空间增加，与阴离子头的大小相匹配，表面活性剂膜变得紧密。LC ₁₂ PO ₁₅ S 在 0.5% NaCl 对癸烷的情况下出现超低 IFT 值。添加二价阳离子可以显着降低 IFT。随着 Ca ²⁺浓度的增加，APS 溶液对烷烃的 IFT 可以在较低的 PO 数和对具有宽 ACN 范围的烷烃时达到超低值。对于MgCl ₂体系，Mg ^{2+ 的}水合半径较小提高了阴离子头部的紧密度，导致油相侧PO链的排列拥挤。因此，Mg ²⁺降低对烷烃的IFT的能力比Ca ²⁺弱。此外，原油中的表面活性物质竞争性吸附在界面上，导致 IFT 增加。通过加入Ca ²⁺或Mg ²⁺，表面活性剂分子会形成紧密的吸附膜，对原油的IFT值达到超低。本文的研究对于在高盐储层条件下应用扩展的表面活性剂溶液来提高石油采收率具有重要意义。